•Highly nitrated cubanes (cube-shaped C8 arrangements) show great promise in explosive applications, being both stable (insensitive to shock) and more powerful than HMX, one of the standard military explosives, and even an experimental compound, CL-20, usually cited as the most powerful non-nuclear explosive.
•The primary difficulty comes in the synthesis of the nitrated cubanes; there is little nitration research done on the kind of system being nitrated, a saturated cyclic molecule, and there is even less on systematic nitration. Developing a pathway and a process for this reaction was the researchers’ aim.
•Their prior research has yielded 1,3,5,7-tetranitrocubane, mainly by replacing some of the hydrogens with amine groups and then replacing those with the nitro groups. The method used to get to the tetranitrocubane, beginning with a carboxylated cubane, is one they can’t replicate on a “larger” scale, because the cubane bonds would end up cleaving; some of the group intermediates are electron-withdrawing, and the cubane molecule is already very strained.
•The group got to the pentanitrocubane and the hexanitrocubane by nitrating the tetranitrocubane at a THF/N2O4 interface. This worked well, but they would need to find a different methodology to be able to successfully get heptanitrocubane and to be able to more readily replicate the results.
•For heptanitrocubane, they found a procedure that worked a bit more readily: treat the tetranitrocubane with NaN(TMS)2 in a THF/(CH3)-THF solution around -78˚C to generate the reactive monoanion. The system was cooled further (to around -130˚C), and N2O4 in isopentane was added. The reaction was likely stopped by the addition of nitric acid in diethyl ether, and by the addition of the entire solution to water. The amount of the NaN(TMS)2 impacted the extent of the nitration that happened; at high enough concentrations, almost all of the cubane had become heptanitrocubane, with about a 74% yield. There was no octanitrocubane, though.
•Analysis of heptanitrocubane showed some interesting properties. It is soluble in polar solvents, for one. Also, it has a density of 2.028 g/cm³, a number that is pretty high for a compound only composed of C, H, N, and O. The bond distance between C atoms is about 1.561 Å, and some hydrogen bonding is the result of the lone hydrogen.
•Heptanitrocubane is apparently acidic, forming a yellow complex with the addition of deuterated methanol and readily taking on iodine. It also decomposes and deflagrates in the presence of a base, such as pyridine or NaF.
•Alkali metal salts were prepared, though these salts decomposed very readily as temperatures increased. The use of powerful electrophiles results in the replacement of the lone hydrogen on the heptanitrocubane with a desired group.
•The salts, though, seem to be of little use in attempting to generate octanitrocubane; common electrophilic reagents failed to produce the replacement reaction. The suggested reason for this failure is the relative stability of the heptanitrocubane anion.
•Octanitrocubane was finally produced by adding excess NOCl to the lithium salt of heptanitrocubane, followed up by ozonation, at -78˚C; the yield was 45-55%, and there was an unstable intermediate that apparently broke down shortly after formation.
•Octanitrocubane is a stable, crystalline solid that also dissolves in polar solvents. The bond length for the C atoms is largely unchanged, though the density is actually a good bit lower than initially predicted (1.979 g/cm³, compared to a predicted value of at least 2.123 g/cm³). There may be a denser form of octanitrocubane than the one the scientists initially obtained; the theoretical efficacy of the compound as an explosive is based off of density, and the density obtained is not what they were hoping for.
•In 13C-NMR analysis of octanitrocubane, a triplet peak actually resulted around 87.8 ppm, the result of 13C-14N coupling. Once decoupled for this, the spectrum indeed shows a sole singlet peak. Carbon-nitrogen coupling is rare, though it shows up when there is a very strong electron-withdrawing group present, giving charge symmetry around the nitrogen nucleus.
[edit 11/29/2012: addition of article summary]
[This article was gotten off of the Interlibrary Loan. I'd be happy to supply a link or an upload, however that needs to be done.]
Article Citation
Zhang, Mao-Xi, Phillip Eaton, and Richard Gilardi. “Hepta- and Octanitrocubanes.” Angewandte Chemie, Int. Ed. 2000, 39, no.2.ILLiad link to the article
Article Summary
•Highly nitrated cubanes (cube-shaped C8 arrangements) show great promise in explosive applications, being both stable (insensitive to shock) and more powerful than HMX, one of the standard military explosives, and even an experimental compound, CL-20, usually cited as the most powerful non-nuclear explosive.
•The primary difficulty comes in the synthesis of the nitrated cubanes; there is little nitration research done on the kind of system being nitrated, a saturated cyclic molecule, and there is even less on systematic nitration. Developing a pathway and a process for this reaction was the researchers’ aim.
•Their prior research has yielded 1,3,5,7-tetranitrocubane, mainly by replacing some of the hydrogens with amine groups and then replacing those with the nitro groups. The method used to get to the tetranitrocubane, beginning with a carboxylated cubane, is one they can’t replicate on a “larger” scale, because the cubane bonds would end up cleaving; some of the group intermediates are electron-withdrawing, and the cubane molecule is already very strained.
•The group got to the pentanitrocubane and the hexanitrocubane by nitrating the tetranitrocubane at a THF/N2O4 interface. This worked well, but they would need to find a different methodology to be able to successfully get heptanitrocubane and to be able to more readily replicate the results.
•For heptanitrocubane, they found a procedure that worked a bit more readily: treat the tetranitrocubane with NaN(TMS)2 in a THF/(CH3)-THF solution around -78˚C to generate the reactive monoanion. The system was cooled further (to around -130˚C), and N2O4 in isopentane was added. The reaction was likely stopped by the addition of nitric acid in diethyl ether, and by the addition of the entire solution to water. The amount of the NaN(TMS)2 impacted the extent of the nitration that happened; at high enough concentrations, almost all of the cubane had become heptanitrocubane, with about a 74% yield. There was no octanitrocubane, though.
•Analysis of heptanitrocubane showed some interesting properties. It is soluble in polar solvents, for one. Also, it has a density of 2.028 g/cm³, a number that is pretty high for a compound only composed of C, H, N, and O. The bond distance between C atoms is about 1.561 Å, and some hydrogen bonding is the result of the lone hydrogen.
•Heptanitrocubane is apparently acidic, forming a yellow complex with the addition of deuterated methanol and readily taking on iodine. It also decomposes and deflagrates in the presence of a base, such as pyridine or NaF.
•Alkali metal salts were prepared, though these salts decomposed very readily as temperatures increased. The use of powerful electrophiles results in the replacement of the lone hydrogen on the heptanitrocubane with a desired group.
•The salts, though, seem to be of little use in attempting to generate octanitrocubane; common electrophilic reagents failed to produce the replacement reaction. The suggested reason for this failure is the relative stability of the heptanitrocubane anion.
•Octanitrocubane was finally produced by adding excess NOCl to the lithium salt of heptanitrocubane, followed up by ozonation, at -78˚C; the yield was 45-55%, and there was an unstable intermediate that apparently broke down shortly after formation.
•Octanitrocubane is a stable, crystalline solid that also dissolves in polar solvents. The bond length for the C atoms is largely unchanged, though the density is actually a good bit lower than initially predicted (1.979 g/cm³, compared to a predicted value of at least 2.123 g/cm³). There may be a denser form of octanitrocubane than the one the scientists initially obtained; the theoretical efficacy of the compound as an explosive is based off of density, and the density obtained is not what they were hoping for.
•In 13C-NMR analysis of octanitrocubane, a triplet peak actually resulted around 87.8 ppm, the result of 13C-14N coupling. Once decoupled for this, the spectrum indeed shows a sole singlet peak. Carbon-nitrogen coupling is rare, though it shows up when there is a very strong electron-withdrawing group present, giving charge symmetry around the nitrogen nucleus.